1. Field of the Invention
The present invention relates generally to printing of viscous materials onto a surface through a screen having a plurality of openings therein, and more particularly to method and apparatus for cleaning paste residues from screens used in producing surface mount technology circuit boards suitable for use in high technology products.
2. Description of the Prior Art
Screens used in producing such circuit boards require frequent cleaning because viscous printing material, including solder paste, is caused to remain not only on screen surfaces, but also in openings in the screen during repeated printing. If the printing is further continued, the remaining solder paste as paste residue will cause defective printing such as blot, blur, discontinuity of line, or the like, of a paste pattern printed on the circuit board. On the other hand, to meet the rapid development of the electronics industry, a screen is required to obtain high resolution. That is, the area of opening pattern on the screen and the space between adjacent openings in the screen tend to be reduced. Such a tendency makes it difficult to effectively remove solder paste residues from the screen, particularly the residues that adhere to inner peripheral surfaces of small or reduced openings in the screen.
There are various methods and apparatuses by which solder paste residues that are adhered to the screen can be removed. One approach is to wipe off the residues by applying paper to the screen surfaces. Another approach is to scrape off the residues by scraper means. However, it is impossible for these approaches to remove the residues in the openings in the screen. Another approach is to use an airstream. In this approach, the residues in the openings can be relatively easily removed when the residues are soft. In this approach, however, when the residues adhered to the surfaces of the openings in the screen are solidified or dried, or when the residue is clogged in a small-sized opening in the screen, it is extremely difficult to clean them from the screen.
To overcome drawbacks or disadvantages indicated by these approaches, methods and apparatuses for cleaning screens by using a water-based solvent as a wash liquid have recently been proposed. With this type of method for cleaning a screen, most of the above-mentioned drawbacks or disadvantages can be eliminated. However, the use of a cleaning solvent causes environmental and personal injury resulting from solvent release into the atmosphere, and worker exposure to solvent vapors. Furthermore, solder paste residue contains lead that is removed from the screen during cleaning, and worker exposure to lead is a health concern which must be seriously addressed.
A known apparatus of this type is schematically shown as PRIOR ART 1 in FIG. 11. As shown therein, the known apparatus is provided with a cleaning reservoir 103 with its upper end opened and has therein two inwardly directed mutually opposed spray nozzles 131 for spraying cleaning solvent against both surfaces of the screen 105 which is vertically held between the nozzles 131 during cleaning. Numeral 132 shown in FIG. 11 indicates drier nozzles. Another known apparatus of the type mentioned above is also schematically shown as PRIOR ART 2 in FIG. 12. As shown therein, the apparatus is provided with a box-like housing 200 having an opening at its upper wall through which a screen 205 is vertically inserted into the housing 200 for cleaning. In the housing 200 there are provided two spray nozzles 201, 202 for spraying the solvent against both surfaces of the screen 205, which is vertically held between the nozzles 201, 202 during cleaning. The upper opening of the housing may be closed and opened and is closed in the manner shown in FIG. 12 during cleaning. Numerals 203, 204 show drier nozzles for drying the screen 205 after spraying is finished.
A common feature of these known apparatuses is that the screen is held in an erect position during cleaning. Other common features are that the apparatus is installed outside a screen printing machine, that the apparatus becomes bulky as a whole, due to its abovementioned structures, and cannot be incorporated into a screen printing machine, and, accordingly, that it cannot be used for cleaning of the screen during paste printing cycles performed by the machine.